1. Field of the Invention
The present invention relates to an equivalent time sampling radar, and more particularly to a cancellation of interference between radars, or a cancellation of an interference wave from an electric wave of other broadcasting or that of other communication.
2. Description of the Related Arts
In a general equivalent time sampling radar, a distance is calculated on the basis of a roundtrip propagation time that is a time from when a transmission pulse signal is reflected on a target, which is a subject to the distance-measuring, to when the reflection signal is received. When another radar is present in the distance-measuring range of the above general radar, it is difficult for the general radar to determine whether the reception pulse signal is the signal (reflection wave) transmitted from the own radar or the signal (interference wave) transmitted from another radar, which causes the deterioration in the distance-measuring accuracy. When the timing of the sampling is synchronous with the electric wave of another broadcasting, that of communication, or the like, there is a high possibility that interference occurs between the reception signal and the signal transmitted from another radar.
JP-A-2005-24563 discloses a short-range vehicular radar system including, in a programmable digital receiver, a non-linear filter (adaptive interference filter) for rejecting a radar pulse received from another radar. Specifically, the receiver cancels the interference wave (i.e., the transmission pulse transmitted from another radar) by the programmable non-linear filter. Thus, the pulse that appears to be statistically much larger than other pulses received by the receiver is cancelled.
JP-A-2006-177985 discloses a technique capable of preventing a malfunction due to the interference from another radar by using a pseudo-random signal generator in a pulse radar apparatus with a short-range detection limit and a high distance resolution. Specifically, a signal is generated by using a first signal that is a basic signal in generating a transmission pulse, a second signal for phase modulation of a frequency lower than the first signal, and a pseudo-random signal, for amplitude modulation, generated at an intermediate frequency between the first and second signals, and the generated signal is delayed, whereby a control pulse signal is generated. A gate operation for the reception signal is performed based upon the control pulse signal. A noise band can be restricted by the cutoff using a narrow band filter, and a malfunction of detecting wrong targets by the interference of other radar units can be suppressed by the spread spectrum system.
However, in JP-A-2005-24563 and JP-A-2006-177985, the transmission timing of the transmission pulse or the sampling timing of the reception pulse depends upon a reference clock, thereby entailing a problem that a satisfactory effect of reducing the interference cannot be expected for electric waves transmitted at the frequency or timing that happens to be synchronous with the reference clock (the interference wave that is the subject to be treated in JP-A-2005-24563 and JP-A-2006-177985 is not the one described above to begin with)